Plastic injection molding is a very large industry, and we rely on hundreds of plastic injection molded products every day. The list is endless, but includes interior automotive parts, cell phone cases and windows, caps and closures, children's toys, kitchen items, office equipment, etc. Even though great advances have been made in this industry, product complexity and production rates are still limited. These limitations do not result from the injection molding machines themselves, but rather from the restrictions imposed by tooling design and tooling materials used in the injection molding machines.
The known tools for injection molding machines are made from cast and wrought steel and have been available for over 50 years. The known injection molding tools exhibit a number of disadvantages. The known machines are designed so that air can vent from the mold cavity as it is filled with the plastic material. The tools are usually inefficiently cooled by water passing through channels drilled into the mold walls. Production efficiency is low because the plastic component must remain in the mold until it is solid enough to hold its shape and be ejected. Other features such as surface quality can also be impaired by cooling inefficiencies. Better and more consistent cooling of the mold would mean that the plastic solidifying temperature could be reached sooner, thus reducing cycle time, and increasing productivity.
Based on current knowledge of gas and gas usage, polymer processing techniques, and the metallurgy of tool materials, a new cooling technology has been developed which has been described as a major breakthrough for the plastic forming industry and especially for plastic injection molding. This technology holds promise for increased productivity, greater design freedom for complex products, increased profits and a widened product scope because it has the potential to reduce production cycle times by 20 to 40%.
The new technology is an advanced cooling concept in which liquid cooling gas is injected into the mold where it evaporates. The resulting gas vents from the mold. This technology requires a mold material manufactured with uniform and controlled porosity to effect optimum cooling. By using micro pores in the mold material, the evaporative cooling points can be situated close to the forming surface of the mold. There is no restriction on the geometry of the mold in the same way as molds made with drilled water channels.
While the technology of gas cooling is viable, the difficulty has been the availability of a suitable material for the molds and the inability to obtain a controlled and consistent quality level of microporosity in the mold material. Further, no efficient technique was available to manufacture a composite mold with a porous interior and a solid surface layer, as is required in many applications where surface quality is of paramount importance.
Attempts to manufacture such tooling by conventional powder metallurgy press-and-sinter techniques have failed to produce the desired results. Those techniques cannot produce the required porosity levels with the degree of control required. Moreover, the press-and-sinter technique produces a non-uniform pore size in the tool material. With this type of variation, the cooling of the tool surface cannot be controlled, and the positives of the new gas cooling technique cannot be effectively utilized. Further, conventional powder metallurgy press and sinter technology cannot produce a solid surface on the tool, while at the same time producing a controlled and consistent microporous substructure.
It is clear that a material and process needs to be developed to manufacture tooling that can take advantage of the new gas cooling concept for plastic injection molding tools and molds. The process according to the present invention involves the utilization of corrosion resistant materials in combination with a powder metallurgy consolidation process that can produce a microporous tool with controlled porosity and pore size. Further, the consolidation technology according to this invention can produce composite tools designed to have both solid surfaces and porous areas as required for a particular application. Unlike other powder metallurgy consolidation processes, the process according to the present invention provides higher pressure, lower consolidation temperatures, and lower cycle times by orders of magnitude. These advantages result in lower manufacturing costs and significantly lower capital investment.